1. Field of the Invention
The present invention relates to a composite material for a negative electrode, a method for fabricating the same and an electrochemical device using the same and, more particularly, to a composite material for a negative electrode suitable for a nickel-iron battery, a method for fabricating the same and an electrochemical device using the same.
2. Description of Related Art
In the aspect of growing concerns about serious environmental problems, more eco-friendly electric vehicles and hybrid electric vehicles have been developed enthusiastically. Accordingly, large-scale batteries functioning as energy sources are critical for the development of electric vehicles and hybrid electric vehicles. By reviewing the history of the batteries functioning as energy sources for electric vehicles and hybrid electric vehicles, it is known that the nickel-iron batteries were applied in transportation equipments in 1910's. They are suitably functioned as large-scale batteries due to their advantages, such as long life, tolerance of overcharge and overdischarge, abundant material sources and causing no contamination. A nickel-iron battery mainly includes: an iron electrode as a negative electrode, containing iron powders or iron oxides as main components; a nickel hydroxide electrode as a positive electrode; an alkaline electrolyte, usually being a mixed aqueous solution of potassium hydroxide and lithium hydroxide; and a separator, interposed between the negative electrode and the positive electrode.
The iron electrode using iron powders as main components are usually prepared by atomization of molten metal, electrolysis, reduction of magnetite by carbon, decomposition of Fe(CO)5 or reduction of iron oxide powders by hydrogen gas. Among the above-mentioned methods, hydrogen reduction of fine iron oxide powders is more usually used to obtain porous fine iron powders. Alternatively, the technology of using Fe3O4 in an electrode has been developed. For example, the Fe3O4 active material with no treatment of hydrogen reduction can be prepared into an electrode; or ferrous oxalate is first heated up to 200° C. to remove water, then the water-free ferrous oxalate is heated up to 500° C. to perform decomposition and thereby to obtain products consisting of 15 wt. % of α-Fe and 85 wt. % of Fe3O4, and finally the products are prepared into an electrode. The capacity of such electrode is about 220 mAh/g under 40 mA/g.
However, the iron electrodes fabricated by these conventional methods have the drawback of poor capacity. Thereby, when these iron electrodes are applied in batteries as power sources, the energy density and power density of such entire batteries cannot satisfy the requirements. Accordingly, it is desirable for the present invention to improve capacity and charge/discharge characteristics of a battery.